1. Field of the Invention
The present invention relates to a band-pass filter circuit that is installed in a communication device.
2. Description of Related Art
In recent years, as a communication device, a mobile phone using the frequencies of an 800 MHz band and a 2 GHz band has been popular. Also, as the communication device, a wireless LAN using the frequencies of a 2 GHz band and a 5 GHz band has been popular. So, in order to remove the frequency component, which is caused by noise and interference, from the input frequency of the input signal supplied to the communication device, the communication device includes a band-pass filter circuit. The band-pass filter circuit passes a passband including a band-pass center frequency with respect to the input frequency and cuts off a cutoff frequency band other than the passband. The band-pass filter circuit is especially required to exhibit a small loss (low insertion loss) in the pass characteristic when the band-pass center frequency is passed.
As a band pass filter circuit in a related art, Japanese Laid-Open Patent Application JP-P2004-248121A discloses a band pass filter. FIG. 1 shows the band pass filter circuit disclosed in this related art.
The band pass filter circuit includes: resonance lines 101, 102 in which one ends are grounded and they are coupled to each other and resonate at a band-pass center frequency; a parallel resonant circuit 111 connected between the other end of the resonance line 101 and an input end (a first port P1); and a parallel resonant circuit 112 connected between the other end of the resonance line 102 and an output end (a second port P2). Each of the parallel resonant circuits 111, 112 has an inductor and a capacitor that are connected in parallel to each other. This is characterized in that the resonant frequency of the parallel resonant circuits 111, 112 is set to be lower than the band-pass center frequency. Consequently, interference signals lower than the band-pass center frequency can be suppressed by the parallel resonant circuits 111, 112.
Moreover, according to the disclosed band pass filter circuit, a capacitor C101 is connected between the other ends of the resonance lines 101, 102. This is characterized in that the parallel resonant frequency, which results from the resonance lines 101, 102 and the capacitor C101, is set to be higher than the band-pass center frequency. Consequently, a pass bandwidth can be adjusted by the parallel resonant circuits 111, 112 and the capacitor C101.
In the band pass filter circuit, the resonance lines 101, 102 that resonate at the band-pass center frequency must be coupled to each other. For this reason, usually, it is not preferable that a line having a length of λ(lambda)/4 is drawn around, because an area of the circuit becomes large, especially in a several GHz band. If a spiral inductor and the like are used in order to make the area small, for example, the couplings must be performed in the upper and lower portions of a multilayer substrate.
In the disclosed band pass filter circuit, the resonance lines 101, 102 have the lengths that the resonance lines resonates at the band-pass center frequency. Thus, at the frequency higher than the parallel resonant frequency, which results from the resonance lines 101, 102 and the capacitor C101, the disclosed band pass filter circuit has a disadvantage that the impedance is high, the ground property is poor, and the strong attenuation is not obtained.
FIG. 2 is a view showing a propagation characteristic of the band pass filter circuit in the related art. In FIG. 2, insertion losses dB (S(2,1)) at the frequency of 1.8 GHz, 2.5 GHz and 3.5 GHz are represented by markers m13, m6 and m15, respectively. The insertion loss dB (S(2,1)) is represented as, for example, −20×log S(2,1) [dB] by using a band-pass characteristics S(2,1). The band-pass characteristics S(2,1) is represented as (Reflection electric power of second port P2)/(Input electric power of first port P1) when the input electric power of the second port P2 is assumed to be 0 (zero).
In the band pass filter circuit in the related art, the insertion loss of the band-pass center frequency of 2.5 GHz is 0.786 dB, the insertion loss of a lower cutoff frequency of 1.8 GHz is 67.1 dB, and the insertion loss of a higher cutoff frequency of 3.5 GHz is 24.1 dB. In addition, even in the other frequency bands, they are approximately 15 dB or more (−15 dB or less).
In this way, in the communication device that includes the band pass filter circuit, if the frequency component caused by the noise and the interference can be removed over that of the technique in the related art, the reliability on a communication is further improved. Thus, it is desired that the band pass filter circuit where the properties of the passband with a low insertion loss and the cutoff frequency band with a high isolation to be required are superior to the properties of the technique in the related art.
In addition to the above-mentioned related art, techniques in other related arts will be introduced below.
Japanese Laid-Open Patent Application JP-P 2005-328262 A describes a filter. In the filter, a plurality of parallel branch parts and a plurality of direct branch parts, which are constituted by resonant circuits, are alternately connected, and the resonant circuit of at least one direct branch part is configured such that two parallel resonant circuits are connected in series. The filter is characterized in that an inductive resonant circuit is installed between a conductor to connect the two parallel resonant circuits connected in series and a grounded conductor of the filter. Consequently, the influence caused by the capacitive component of the conductor to connect the two parallel resonant circuits is reduced in the filter.
Japanese Laid-Open Patent Application JP-A-Heisei, 8-148958 describes a filter circuit. The filter circuit is the Chebyshev-type filter circuit where a plurality of LC parallel resonant circuits are connected in parallel between an input terminal and an output terminal. The filter circuit is characterized in that at least one condenser to form a pole in an attenuation band is connected to a predetermined position. Consequently, the number of the inductances can be reduced, and the sharp attenuation characteristic can be obtained.